The present invention relates to an electric motor, in particular for power tools, whose stator has at least one pole shoe covering a portion of the rotor, and a brake element that is movable in the direction of the rotor is disposed in the pole shoe and is provided with a brake lining.
Particularly in power tools, such as angle grinders, circular saws, and the like, after the motor current is switched off, the rundown times are sometimes quite long, because of a rotational energy stored in the rotating tool.
The tool is still rotating after being switched off presents a risk of injury to the user. To reduce the rundown times of power tools, it is known, for instance from German Patent Disclosure DE 198 60 396 A1 to provide a brake element in the electric motor, which develops a braking action on the rotor as soon as the current to the engine has been switched off. A very space-saving braking device which achieves good braking action is the subject of German Patent Disclosure DE 198 60 396 A1. This known braking device comprises a brake element, embodied as a rocker, which is suspended in a radially continuous free space in a pole shoe of the stator of the motor. With this kind of brake element embodied as a rocker, a very high braking force acting on the rotor can be generated because of the leverage of the rocker. The brake element has a brake lining. According to DE 198 60 396 A1, this brake lining should comprise a material with no, only a very slight, magnetic conductivity. This kind of brake lining, located in the air gap between the rotor and the stator of the motor, presents a strong magnetic resistance, compared to the pole shoe, and this attenuates the magnetic flux in the air gap. The consequence is a relatively high idling rpm of the motor, yet this is especially unwanted in power tools and with a long tool radius, such as in angle grinders. To reduce the idling rpm, steps would have to be taken that would reduce the power of the motor, which is once again unwanted.
In an electric motor of the invention, the brake element is provided with a brake lining, which has approximately the same magnetic conductivity as the pole shoe and the brake element inserted into it. As a result, in the air gap between the stator and the rotor of the motor, the brake lining does not present any increased magnetic resistance that impairs the exciter flux. Hence an unwanted high idling rpm of the motor also is averted.
The brake lining advantageously comprises a substrate material filled with iron powder.
It is expedient that the brake element is embodied as a rocker, which is suspended in a radially continuous free space in the pole shoe. By the leverage of the rocker, a strong braking force for the rotor of the motor can be generated. The free space is preferably defined axially by a respective rib.
The brake element is supported pivotably about a pivot shaft within limits relative to the stator, and the pivot shaft is located parallel to an axis of rotation of the rotor. The brake element can be provided laterally with one peg each, which engages respective recesses of the ribs and forms the pivot shaft.
It is expedient that the brake element is embodied as a multi-armed lever, with a braking arm and a disengagement arm, and the braking arm is subjected to a braking force and the disengagement arm, when current is supplied to the stator winding, exerts a disengagement force, oriented counter to the braking force, on the brake element.
Preferably, a parting face between the disengagement arm and the pole shoe has a greater radial spacing from the pivot shaft than a parting face between the braking arm and the pole shoe.
In an advantageous embodiment, the braking arm has at least one blind bore, which serves to receive a compression spring, and the compression spring is advantageously braced on a brace secured to the stator.
Vibration of the braking device when the stator controller is supplied with current can advantageously be prevented by providing that a short-circuit winding is integrated into the disengagement arm of the brake element.